The present invention relates to a hydraulic drive system having a pressure-regulated hydraulic pump for the provision of a system pressure and having a secondarily regulated hydraulic motor.
In such secondarily regulated hydraulic drive systems, the hydraulic pump which is made as a variable delivery pump provides, with the help of a pressure regulation, a system pressure in the line network which is applied to the hydraulic motor. The hydraulic motor which is made as a variable capacity motor then drives a consumer with speed regulation, torque regulation and/or rotational angle regulation. Such a secondarily regulated hydraulic drive system thus includes a pressure-regulated variable delivery pump as a primary unit and a variable capacity motor with speed regulation, torque regulation and/or rotational angle regulation as a secondary unit. A mechanical transmission can be connected before such a hydraulic drive system and connects a drive unit such as an internal combustion engine to the hydraulic pump.
The following designs of the components usually result for a secondarily regulated hydraulic drive system:                Axial piston units in swash plate construction are usually used for the setting up of a simple secondarily regulated propulsion drive, with any other adjustable hydraulic displacement machines, however, also being able to be used for the hydraulic motor and the hydraulic pump.        The hydraulic pump must be able to be adjusted from 0 to a maximum pumping volume.        The hydraulic motor must be able to be adjusted from a maximum negative displacement volume to a maximum positive displacement volume if the apparatus should be both accelerated and decelerated with the help of the hydraulic drive system. The hydraulic motor must therefore be adjustable from 0 both to a maximum negative displacement or pumping volume to a maximum positive displacement or pumping volume.        The pressure-regulated hydraulic pump pumps exactly the oil volume flow into the line network which is required to maintain the system pressure in the line network at a desired system pressure. In this connection, with known secondarily regulated hydraulic drive systems, the system pressure is independent of the oil volume flow removal by the hydraulic motor.        The output torque of the hydraulic motor, which is usually in axial piston construction, is in proportional relation to the set displacement volume and to the applied supply pressure which is provided as the system pressure by the hydraulic pump. The speed regulation of the hydraulic motor then regulates the displacement volume of the hydraulic motor exactly so that its output torque accelerates or decelerates the drivetrain of the apparatus to be driven to the speed desired by the operator or maintains it at the speed. Depending on the displacement volume set, the hydraulic motor removes the required volume flow from the line network. If the speed setting of the operator requires a deceleration of the drivetrain, the hydraulic motor changes its displacement direction and works in pump operation so that it pumps oil volume into the line network and thus brakes the apparatus connected to the hydraulic motor. Equally, in addition to such a speed regulation, a torque regulation or rotational angle regulation of the hydraulic motor is also conceivable.        Advantageously, in this respect, a high pressure store which can store hydraulic energy temporarily is provided in the hydraulic circuit of hydraulic pump and hydraulic motor. If the system pressure increases, for example in that the hydraulic motor works in pump operation, the high pressure store receives oil volume and can then return it to the line network at a later time. With secondarily regulated hydraulic drive systems, a substantial energy saving can hereby be realized with respect to primarily regulated hydraulic drive systems.        
Such secondarily regulated hydraulic drive systems can be used e.g. as a hydrostatic transmission in a propulsion drive of a vehicle, for example for earth-moving machinery. Hydraulic drive systems, in particular hydrostatic propulsion drives, usually have to be able to cover a wide performance range, that is, the drive must satisfy both small performance demands (low speeds and accelerations at low drive torques) and large performance demands (high speeds and accelerations at high drive torques) within preset performance limits.
With hydrostatic propulsion drives, for example, the drive for a typical output cycle requires a high output torque during the start-up phase to accelerate the vehicle, e.g. an earth-moving machine, to the desired speed. The propulsion drive during travel at the reached speed, in contrast, still only requires a low output torque to maintain the machine at the speed. Similar load cycles also result with a plurality of other usage possibilities of hydraulic drive systems.
Since the output torque of a secondarily regulated hydraulic motor is dependent on the product of its displacement volume and the supply pressure, the maximum possible displacement volume of the hydraulic motor as well as the supply pressure must be designed for the maximum load on the drive system. Accordingly, during working phases with lower performance demands, the displacement volume of the hydraulic motor must be reduced by regulation.
Adjustable hydraulic displacement machines, in particular axial piston units, however, in principle have high losses at working points with small displacement volumes and high operating pressures. If the performance demand on the hydraulic drive system is low as is e.g. the case with smooth driving at low speed and low tractive force or when the vehicle is stationary, both displacement units work at precisely such working points, however, in conventional secondarily regulated hydraulic drive systems. Both the pressure regulated hydraulic pump and the secondarily regulated, e.g. speed regulated, hydraulic motor continue to work with the system pressure also provided for maximum performance demands and accordingly with small pumping or displacement volumes at these low performance demands. Both high energy losses, and thus a low total efficiency, and a high load of all components hereby result.